Animal Digestive System Nutrition Design January 15, 2024 BIO 34
Animals obtain their food from other organisms, including plants, other animals, or both.
Herbivores are animals whose primary food sources is plant-based, such as deer, koalas, and some birds and invertebrates. Herbivores have a digestive system that can handle large amounts of plant material. Herbivores can also be divided into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters).
Carnivores are animals that eat other animals, including wild cats, snakes, sharks, some invertebrates and insects. Obligate carnivores depend entirely on animal flesh for their nutrients, such as in the cat family. Facultative carnivores are those animals that eat other animals along with non-animal food.
Omnivores are animals that eat both plant and animal-based food, such as humans, bears, chickens, and some invertebrates and insects.
Invertebrate animal digestive systems help digest foods. The gastrovascular cavity is found in animals with only one opening for digestion, such as flatworms, coral, jellyfish, and sea anemones. Gastrovascular cavities usually include a blind tube or cavity with one opening, that serves as a mouth and an anus. Food enters the mouth and passes through this hollow tube cavity where cells secrete digestive enzymes that breakdown the food. The food particles are then engulfed by the cells of the gastrovascular cavity.
The alimentary canal is a different digestive system containing one tube with a mouth at one end and an anus at the other end (e.g. earthworms). Once the food is ingested through the mouth, it passes through the esophagus and is stored in an organ called the crop. Then the food passes into the gizzard where it is churned and digested. Then, the food passes through the intestine, the nutrients are absorbed, and the waste is eliminated as feces (or castings) through the anus.
Vertebrate animals have different digestive systems with one or more stomach chamber cavities. A single-chambered cavity is called monogastric and is found in humans and many animals. The monogastric digestive process begins when food is ingested in the mouth. Teeth in the mouth help break down the ingested food into smaller pieces. Enzymes in the saliva also help break down the food into smaller pieces. The esophagus is a long tube that connects the mouth to the stomach. Using wave-like smooth muscle contractions called peristalsis, the muscles of the esophagus push the food toward the stomach. Acid in the stomach helps the enzymes work faster. The gastric juices, which include enzymes in the stomach, work on the food particles and continue the digestion process. Further breakdown of food occurs in the small intestine where enzymes produced by the liver, the small intestine, and pancreas continue the process of digestion. The nutrients are absorbed into the bloodstream across the epithelial cells lining the walls of the small intestines. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces and stored until excreted through the rectum.
Avian bird digestion is unique because birds do not have teeth and their digestive systems must be able to process un-chewed food. Birds do have a variety of beaks that can be used to catch food, including seeds, fruits, nuts, and insects. Birds have a high metabolism and eat often to produce energy for flying and reduce weight by quick digestion. The food enters the mouth and long esophagus tube before reaching the crop pouch. Then the food enters the stomach chambers near the liver and pancreas. Bird stomachs have two chambers: the proventriculus chamber, where gastric juices are produced to digest the food before it enters the stomach. The gizzard chamber is where food is stored, soaked, and mechanically ground, but some material is undigested. Most of the chemical digestion and absorption happens in the intestine and the waste is excreted through the cloaca.
Ruminants are mostly herbivore hoofed mammals with like cows, sheep, goats, llamas, deer, camels, giraffes, bison, caribou, and antelope. These animals eat a large amount of roughage (fiber). Ruminant digestive systems allow for digestion of large amounts of fiber or cellulose. Ruminants are unique in that they have no upper front incisor teeth, but have a tough, fibrous dental pad on the roof of their mouths. Lower teeth, tongue, and lips are used to chew food. From the mouth, food travels through the esophagus to the stomach.
The ruminant stomach is a multi-chambered organ that helps digest large amounts of fiber and cellulose material. The four chambers of the stomach are the rumen, reticulum, omasum, and abomasum. These chambers contain many microbes that break down cellulose and ferment ingested food. The abomasum is called the "true stomach" and is the equivalent of the monogastric stomach chamber where gastric juices are secreted. The four compartment gastric chamber provides ample space and microbial support necessary to digest plant material in ruminants. The fermentation process produces large amounts of gas in the stomach chamber, which must be eliminated. As in other animals, the small intestine plays an important role in nutrient absorption, and the large intestine helps in the elimination of waste.
Pseudo-ruminants (camels and alpacas) have a three-chamber stomach digestive system and a large cecum, a pouched organ at the beginning of the large intestine with microorganisms and is where the cellulose plant material is fermented and digested. The rumen is absent, but the omasum, abomasum, and reticulum are present in pseudo-ruminants.
The vertebrate digestive system has an oral cavity or mouth where food is ingested and chewed down or masticated with the teeth. The chemical process of digestion begins in the mouth. During food chewing, saliva is produced by salivary glands and mixes with the food. Saliva is a watery substance produced in the mouths of many animals. Three major glands secrete saliva: the parotid, submandibular, and the sublingual. Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains immunoglobins and lysozymes, which have antibacterial action to reduce tooth decay by inhibiting growth of some bacteria. Saliva also has an enzyme called salivary amylase that begins the process of converting starches in the food into a disaccharide called maltose. Another enzyme called lipase is produced by cells in the tongue. Lipases are a class of enzymes that can break down triglycerides. The lingual lipase begins the breakdown of fat components in the food. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing by moving the bolus from the mouth into the pharynx. The pharynx opens to two passageways: the trachea, which leads to the lungs, and the esophagus, which leads to the stomach. The trachea has an opening called the glottis, which is covered by a cartilaginous flap called the epiglottis. When swallowing, the epiglottis closes the glottis and food passes into the esophagus and not the trachea. This arrangement allows food to be kept out of the trachea.
The esophagus is a tubular organ that connects the mouth to the stomach and transports chewed foot to the stomach. The smooth muscles of the esophagus undergo a series of wavelike movements called peristalsis that push the food toward the stomach in one direction (irreversible). Peristalsis movement of the esophagus is an involuntary reflex that occurs as a result of swallowing food.
A ring-like muscle called the sphincter forms valves in the digestive system at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents stomach contents from traveling up the esophagus. However, acidic digestive juices can escape into the esophagus sometimes and cause acid reflux or heartburn.
The stomach is where a large part of digestion occurs. The stomach is a saclike organ that secretes gastric digestive juices. Stomach pH ranges from 1.5 to 2.5 and this highly acidic environment is required for the chemical breakdown of food and the extraction of nutrients. The stomach can expand up to 20 times in size to allow food to enter and this expansion can help animals that eat large amounts of food when it is available.
In many animals, the stomach is the site of protein digestion. Protein digestion is mediated by an enzyme called pepsin in the stomach chamber. Pepsin is secreted by the chief cells in the stomach in an inactive form called pepsinogen. Pepsin breaks peptide bonds and cleaves proteins into smaller polypeptides. Pepsin also helps activate more pepsinogen, starting a positive feedback mechanism that generates more pepsin. Another cell type called parietal cells secrete hydrogen and chloride ions, which combine in the lumen to form hydrochloric acid, the primary acidic component of the stomach juices. Hydrochloric acid helps to convert the inactive pepsinogen to pepsin. The highly acidic environment also kills many microorganisms in the food and along with the pepsin enzyme, the result is the hydrolysis of protein in the food. Chemical digestion is facilitated by the churning action of the stomach. Contraction and relaxation of smooth muscles mixes the stomach contents about every 20 minutes. The partially digested food and gastric juice mixture is called chyme, which passes from the stomach to the small intestine. Further protein digestion takes place in the small intestine. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of the chyme to the stomach into the small intestine is regulated by the pyloric sphincter.
When digesting protein and some fats, the stomach must be protected from getting digested by pepsin. Since pepsin is originally made in the inactive form pepsinogen, the stomach cells are protected. Also, the stomach has a thick mucus lining that protects the underlying tissue from the action of digestive juices. When this mucus lining is ruptured, wounds called ulcers can form in the stomach caused by bacteria.
Chyme moves from the stomach to the small intestine, which is the organ where digestion of proteins, fats, and carbohydrates is competed. The small intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi. The apical surface of each villus has many microscopic projections called microvilli. These structures are lined with epithelial cells on the luminal side and allow for the nutrients to be absorbed from the digested food and absorbed into the bloodstream on the other side. The villi and microvilli, with their many folds, increase the surface area of the intestine and increase the absorption efficiency of the nutrients. Absorbed nutrients in the blood are carried into the hepatic portal vein, which leads to the liver. There, the liver regulates the distribution of nutrients to the rest of the body and removes toxic chemicals and pathogens.
The human small intestine is over 6 m long and is divided into three parts: the duodenum, the jejunum, and the ileum. The duodenum is the C-shaped, fixed part of the small intestine. The duodenum is separated from the stomach by the pyloric sphincter which opens to allow chyme to move from the stomach to the duodenum. In the duodenum, chyme is mixed with pancreatic juices in an alkaline solution rich in bicarbonate that neutralizes the acidity of chyme and acts as a buffer. Pancreatic juices also contain several digestive enzymes. Digestive juices from the pancreas, liver, and gallbladder, as well as from gland cells of the intestinal wall itself, enter the duodenum. Bile is produced in the liver and stored and concentrated in the gallbladder. Bile contains bile salts which emulsify lipids while the pancreas produces enzymes that catabolize starches, disaccharides, proteins, and fats. These digestive juices break down the food particles in the chyme into glucose, triglycerides, and amino acids. The bulk of chemical digestion of food takes place in the duodenum. Absorption of fatty acids also takes place in the duodenum.
The second part of the small intestine is called the jejunum, where hydrolysis of nutrients is continued while most of the carbohydrates and amino acids are absorbed through the intestinal lining. Some chemical digestion and the bulk of nutrient absorption occurs in the jejunum.
The ileum is the last part of the small intestine where the bile salts and vitamins are absorbed into the bloodstream. Undigested food is sent to the colon from the ileum via peristaltic movements (wave-like muscle contractions). The ileum ends and the large intestine begins at the ileocecal valve. The vermiform (worm-like) appendix is located at the ileocecal valve. The appendix of humans secretes no enzymes and has an insignificant role in immunity.
The large intestine reabsorbs the water from the undigested food material and processes the waste material. The human large intestine is much smaller in length compared to the small intestine but is larger in diameter. The large intestine has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter. The colon is home to many bacteria or "intestinal flora" that aid in the digestive processes. The colon can be divided into four regions: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material. Carnivorous mammals have a shorter large intestine compared to herbivorous mammals due to their diet.
The rectum is the terminal end of the large intestine and its primary role is to store the feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far end of the digestive tract and is the exit point for the waste material. Two sphincters between the rectum and anus control elimination: the inner sphincter is involuntary and the outer sphincter is voluntary.
Accessory organs of the digestive system add secretions (enzymes) that catabolize food into nutrients. Accessory organs include salivary glands, the liver, pancreas, and the gallbladder. The liver, pancreas, and gallbladder are regulated by hormones in response to the food consumed.
The liver is the largest internal organ in humans and it plays a very important role in digestion of fats and detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fatty components of the food in the duodenum. The liver also processes the vitamins and fats and synthesizes many plasma proteins.
The pancreas is another important gland that secretes digestive juices. The chyme produced from the stomach is highly acidic in nature. The pancreatic juices contain high levels of bicarbonate, an alkali that neutralizes the acidic chyme. Additionally, the pancreatic juices contain a large variety of enzymes that are required for the digestion of protein and carbohydrates.
The gall bladder is a small organ that aids the liver by storing bile and concentrating bile salts. When chyme containing fatty acids enters the duodenum, the bile is secreted from the gallbladder into the duodenum.
Nutrition and energy production in animals is needed for survival and animal nutrition is the source of materials needed for building DNA and other complex molecules needed for growth, maintenance, and reproduction. These processes are known as biosynthesis. Food source gives materials needed for ATP production in the cells. Animal diet must be balanced in order to provide vitamins and minerals needed for cellular functions.
Diet in humans and animals must be well balanced to provide nutrients needed for life, good health, and reproductive capability. Humans need fruits, vegetables, grains, proteins, and dairy.
Food sources provide necessary organic molecules for cells and tissues in the body. Carbohydrates or sugars are the primary source of organic carbons in the animal body. During digestion, digestible carbohydrates are broken down into glucose and used to provide energy through metabolic pathways. Complex carbohydrates, including polysaccharides, can be broken down into glucose through biochemical modification. However, humans do not produce the enzyme cellulase and lack the ability to derive glucose from polysaccharide cellulose. In humans, these molecules provide the fiber required for moving waste through the large intestine and a healthy colon. The intestinal flora in the human gut are able to extract some nutrition from these plant fibers. The excess sugars in the body are converted to glycogen and stored in the liver and muscles for later use. Glycogen stores are used to fuel prolonged exertions, such as long distance running, and to provide energy during food shortage. Excess glycogen can be converted to fats, which are stored in the lower layer of skin of mammals for insulation and energy storage. Excess digestible carbohydrates are stored by mammals in order to survive famine and aid in mobility.
Nitrogen is another requirement for nutrition. Protein catabolism provides a source of organic nitrogen. Amino acids are the building blocks of proteins and protein breakdown provides amino acids that are used for cellular function. The carbon and nitrogen derived from these become the building block for nucleotides, nucleic acids, proteins, cells, and tissues. Excess nitrogen must exerted because it is toxic. Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy because one gram of fat contains nine calories. Fats are required in the diet to aid in the absorption of fat-soluble vitamins and the production of fat soluble hormones.
Essential nutrients cannot be produced by the body or produced in enough quantity needed, and therefore these nutrients must be obtained from food outside the body. The omega-3 alpha-linolenic acid and the omega-6 linoleic acid are essential fatty acids needed to make some membrane phospholipids. Vitamins are another class of essential organic molecules that are required in small quantities for many enzymes to function and, for this reason, are considered to be coenzymes. Fat-soluble vitamins and water-soluble vitamins must be obtained from food. Minerals are inorganic essential nutrients that must be obtained from food. Minerals help in structure and regulation and are considered cofactors. Certain amino acids are essential must be obtained from outside the body. Of 20 required amino acids, the human body can only make 11 of these.
Water-soluble Essential Vitamins
Vitamin B1 (Thiamine)
Vitamin B2 (Riboflavin)
Vitamin B3 (Niacin)
Vitamin B5 (Pantothenic acid)
Vitamin B6 (Pyridoxine)
Vitamin B7 (Biotin)
Vitamin B9 (Folic acid)
Vitamin B12 (Cobalamin)
Vitamin C (Ascorbic acid)
Fat-Soluble Essential Vitamins
Vitamin A (Retinol)
Vitamin D
Vitamin E (Tocopherol)
Vitamin K (Phylloquinone)
Minerals Needed for the Body
Calcium
Chlorine
Copper
Iodine
Iron
Magnesium
Manganese
Molybdenum
Phosphorus
Potassium
Selenium
Sodium
Zinc
Essential amino acids that must be consumed
isoleucine
leucine
lysine
methionine
phenylalanine
tryptophan
valine
histidine
threonine
arginine
Essential amino acids anabolized by the body
alanine
selenocysteine
aspartate
cysteine
glutamate
glycine
proline
serine
tyrosine
asparagine
Animals need food to obtain food and maintain homeostasis, which is the ability of a system to maintain a stable internal environment even in the face of external changes to the environment, particularly body temperature.
The primary source of energy for animals is carbohydrates, mainly glucose. The carbohydrates are converted to glucose in the body through a series of biochemical reactions.
Adenosine Triphosphate, ATP, is the primary energy currency in cells. ATP stores energy in phosphate ester bonds. ATP releases energy when phosphodiester bonds are broken and ATP is converted to ADP and a phosphate group. ATP is produced by the oxidative reactions in the cytoplasm and mitochondrion of the cell, where carbohydrates, proteins, and fats undergo a series of metabolic reactions collectively called cellular respiration. For example, glycolysis is a series of reactions in which glucose is converted to pyruvic acid and some of its potential energy is converted to NADH and ATP.
ATP is required for all cellular functions. It is used to build the organic molecules that are required for cells and tissues. It provides energy for muscle contraction and for the transmission of electrical signals in the nervous system. When the amount of ATP is available in excess of the body's requirements, the liver uses the excess ATP and excess glucose to produce molecules called glycogen. Glycogen is a polymeric form of glucose and is stored in the liver and skeletal muscle cells. When blood sugar drops, the liver releases glucose from the stores of glycogen. Skeletal muscle converts glycogen to glucose during intense exercise. The process of converting glucose and excess ATP to glycogen and the storage of excess energy helps animals deal with mobility, food shortages, and famine.
Obesity is not healthy for humans and most animals, but some animals benefit from obesity, such as polar bears when food is scarce.
Digestive System Processes
Ingestion is the first step in the digestive process of breaking down food into smaller pieces in order to access the nutrients. Ingestion is taking in food from the mouth. In vertebrates, the teeth, saliva, and tongue play important roles in mastication (preparing food into bolus). As food is being mechanically broken down, the enzymes in saliva begin to chemically process the food as well. These processes break down food from large particles to soft mass that can be swallowed and travel the length of the esophagus.
Digestion and Absorption
Digestion is the mechanical and chemical breakdown of food into small organic fragments. Breakdown of macromolecules into smaller fragments of suitable size for absorption across the digestive epithelium is important. Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles such as simple sugar before they can be absorbed by the digestive epithelial cells. Different organs play specific roles in the digestive process. The animal diet needs carbohydrates, proteins, fats, as well as vitamins and inorganic components for nutritional balance.
Carbohydrates
The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide. As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucus for lubrication. The acidic environment in the stomach stops the action of the amylase enzyme.
The next step of carbohydrate digestion takes place in the duodenum. Chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver and gallbladder. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide.
The disaccharides are broken down into monosaccharides by enzymes called maltases, sucrases, and lactases, which are also present in the brush border of the small intestinal wall. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose, are broken down by sucrase and lactase, respectively. Sucrase breaks down sucrose (table sugar) into glucose and fructose, and lactase breaks down lactose (milk sugar) into glucose and galactose.
The monosaccharides (glucose) thus produced are absorbed and then can be used in metabolic pathways to harness energy. The monosaccharides are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the body.
Protein
A large part of protein digestion occurs in the stomach. The enzyme pepsin has an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids. In the duodenum, other enzymes like trypsin, elastase, and chymotrypsin act on the peptides reducing them to smaller peptides. Trypsin, elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that break down peptides). Specifically, carboxypeptidase, dipeptidase, and aminopeptidase play important roles in reducing the peptides to free amino acids. The amino acids are absorbed into the bloodstream through the small intestines.
Protein digestion summary:
The liver regulates distribution of amino acids to the rest of the body. In the stomach, pepsin breaks down proteins into fragments, called peptides. Protein-digesting enzymes are secreted from the pancreas into the small intestine. Amino acids are absorbed from the small intestine into the bloodstream. In the small intestine, a variety of enzymes break large peptides into smaller peptides, and then into individual amino acids. A small amount of dietary protein is lost in the feces.
Lipids
Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase. However, the majority of lipid digestion occurs in the small intestine due to pancreatic lipase. When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides by emulsification. Emulsification is a process in which large lipid globules are broken down into several small lipid globules. These small globules are more widely distributed in the chyme rather than forming large aggregates. Lipids are hydrophobic substances: in the presence of water, they will aggregate to form globules to minimize exposure to water. Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts. Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other. By doing so, bile salts emulsify large lipid globules into small lipid globules.
Emulsification is important for digestion of lipids. Pancreatic juices contain enzymes called lipases (enzymes that breakdown lipids). If the lipid in the chyme aggregates into large globules, very little surface area of the lipids is available for the lipases to act on, leaving lipid digestion incomplete. By forming an emulsion, bile salts increase the available surface area of the lipids by many fold. The pancreatic lipases can then act on the lipids more efficiently and digest them. Lipases break down the lipids into fatty acids and glycerides. These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. The bile salts surround long-chain fatty acids and monoglycerides forming tiny spheres called micelles. The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells leaving the micelles behind in the chyme. The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules to become coated with proteins. These large spheres are called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface. The surface is also composed of the hydrophilic phosphate "heads" of phospholipids. Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons leave the absorptive cells via exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood in the subclavian vein.
Vitamins can be either water-soluble or lipid soluble. Fat soluble vitamins are absorbed in the same manner as lipids. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins. Water-soluble vitamins can be directly absorbed into the bloodstream from the intestine.
Elimination is the final step in digestion which involves the elimination of undigested food content and waste products. The undigested food material enters the colon, where most of the water is reabsorbed. The colon is also the home of microflora called "intestinal flora" that aid in the digestion process. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum. As the rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is eliminated through the anus using peristaltic movements of the rectum.
Diarrhea and constipation are some of the most common health problems with elimination. Constipation is a condition where the feces are hardened because of excess water removal in the colon. In contrast, if enough water is not removed from the feces, the result is diarrhea. Many bacteria, including the ones that cause cholera disease, affect the proteins involved in water reabsorption in the colon and result in excessive diarrhea.
Emesis (vomiting) is the elimination of food from the body by forceful expulsion from the mouth. Emesis is often in response to an irritant that affects the digestive tract, including but not limited to viruses, bacteria, emotions, sights, and food poisoning. This forceful expulsion of the food is due to the strong contractions produced by the stomach muscles. The process of emesis is regulated by the medulla (lower brain stem).
Digestive System Regulation
The brain is the control center in animals and humans for the sensation of hunger and satiety. The functions of the digestive system are regulated through neural and hormonal responses.
The first neural response to food is the reaction of smell, sight, or thought of food in the form of salivation. The salivary glands secrete more saliva in response to stimulation by the autonomic nervous system triggered by food in preparation for digestion. Simultaneously, the stomach begins to produce hydrochloric acid to digest the food. Peristaltic movements of the esophagus and other organs of the digestive tract that help food move along are under control of the brain. The brain prepares these muscles for movement as well. When the stomach is full, the part of the brain that detects satiety signals fullness. There are three overlapping phases of gastric control: the cephalic phase, the gastric phase, and the intestinal phase. Each of these requires many enzymes and is under neural control as well.
Digestive phases of gastric control begin with the cephalic phase, which is controlled by the neural response to the stimulus provided by food. Sight, sense, and smell trigger the neural responses resulting in salivation and secretion of gastric juices. The gastric and salivary secretion in the cephalic phase can also take place due to the thought of food. The central nervous system prepares the stomach for food.
The gastric phase begins once food arrives in the stomach and builds on the stimulation provided during the cephalic phase. Gastric acids and enzymes process the ingested materials. The gastric phase is stimulated by distension of the stomach, a decrease in pH of the gastric contents, and the presence of undigested material. This phase consists of local, hormonal, and neural responses. These responses stimulate secretions and powerful contractions.
The intestinal phase begins when chyme (gastric juices and partially digested food) enters the small intestine triggering digestive secretions. This phase controls the rate of gastric emptying. In addition to gastric emptying, when chyme enters the small intestine, it triggers other hormonal and neural events that coordinate the activities of the intestinal tract, pancreas, liver, and gallbladder.
The hormonal responses to food occur as the endocrine system controls the response of the various glands in the body and the release of hormones at the appropriate times.
One of the most important factors under hormonal control is the stomach acid environment. During the gastric phase, the hormone gastrin is secreted by G cells in the stomach in response to the presence of proteins. Gastrin stimulates the release of stomach acid, or hydrochloric acid (HCl) which aids in the digestion of the proteins. However, when the stomach is emptied, the acidic environment need not be maintained and a hormone called somatostatin stops the release of hydrochloric acid. This is controlled by a negative feedback mechanism.
In the duodenum, digestive secretions from the liver, pancreas, and gallbladder play an important role in digesting chyme during the intestinal phase. In order to neutralize the acidic chyme, a hormone called secretin stimulates the pancreas to produce alkaline bicarbonate solution and deliver it to the duodenum. Secretin acts in tandem with another hormone called cholecystokinin (CCK). Not only does CCK stimulate the pancreas to produce the requisite pancreatic juices, it also stimulates the gallbladder to release bile into the duodenum.
Another level of hormonal control occurs in response to the composition of food. Foods high in lipids take a long time to digest. A hormone called gastric inhibitory peptide is secreted by the small intestine to slow down the peristaltic movements of the intestine to allow fatty foods more time to be digested and absorbed.
Hormone control of the digestive system and the role of each hormone is an important area of ongoing research.
Animals obtain their food from other organisms, including plants, other animals, or both.
Herbivores are animals whose primary food sources is plant-based, such as deer, koalas, and some birds and invertebrates. Herbivores have a digestive system that can handle large amounts of plant material. Herbivores can also be divided into frugivores (fruit-eaters), granivores (seed eaters), nectivores (nectar feeders), and folivores (leaf eaters).
Carnivores are animals that eat other animals, including wild cats, snakes, sharks, some invertebrates and insects. Obligate carnivores depend entirely on animal flesh for their nutrients, such as in the cat family. Facultative carnivores are those animals that eat other animals along with non-animal food.
Omnivores are animals that eat both plant and animal-based food, such as humans, bears, chickens, and some invertebrates and insects.
Invertebrate animal digestive systems help digest foods. The gastrovascular cavity is found in animals with only one opening for digestion, such as flatworms, coral, jellyfish, and sea anemones. Gastrovascular cavities usually include a blind tube or cavity with one opening, that serves as a mouth and an anus. Food enters the mouth and passes through this hollow tube cavity where cells secrete digestive enzymes that breakdown the food. The food particles are then engulfed by the cells of the gastrovascular cavity.
The alimentary canal is a different digestive system containing one tube with a mouth at one end and an anus at the other end (e.g. earthworms). Once the food is ingested through the mouth, it passes through the esophagus and is stored in an organ called the crop. Then the food passes into the gizzard where it is churned and digested. Then, the food passes through the intestine, the nutrients are absorbed, and the waste is eliminated as feces (or castings) through the anus.
Vertebrate animals have different digestive systems with one or more stomach chamber cavities. A single-chambered cavity is called monogastric and is found in humans and many animals. The monogastric digestive process begins when food is ingested in the mouth. Teeth in the mouth help break down the ingested food into smaller pieces. Enzymes in the saliva also help break down the food into smaller pieces. The esophagus is a long tube that connects the mouth to the stomach. Using wave-like smooth muscle contractions called peristalsis, the muscles of the esophagus push the food toward the stomach. Acid in the stomach helps the enzymes work faster. The gastric juices, which include enzymes in the stomach, work on the food particles and continue the digestion process. Further breakdown of food occurs in the small intestine where enzymes produced by the liver, the small intestine, and pancreas continue the process of digestion. The nutrients are absorbed into the bloodstream across the epithelial cells lining the walls of the small intestines. The waste material travels on to the large intestine where water is absorbed and the drier waste material is compacted into feces and stored until excreted through the rectum.
Avian bird digestion is unique because birds do not have teeth and their digestive systems must be able to process un-chewed food. Birds do have a variety of beaks that can be used to catch food, including seeds, fruits, nuts, and insects. Birds have a high metabolism and eat often to produce energy for flying and reduce weight by quick digestion. The food enters the mouth and long esophagus tube before reaching the crop pouch. Then the food enters the stomach chambers near the liver and pancreas. Bird stomachs have two chambers: the proventriculus chamber, where gastric juices are produced to digest the food before it enters the stomach. The gizzard chamber is where food is stored, soaked, and mechanically ground, but some material is undigested. Most of the chemical digestion and absorption happens in the intestine and the waste is excreted through the cloaca.
Ruminants are mostly herbivore hoofed mammals with like cows, sheep, goats, llamas, deer, camels, giraffes, bison, caribou, and antelope. These animals eat a large amount of roughage (fiber). Ruminant digestive systems allow for digestion of large amounts of fiber or cellulose. Ruminants are unique in that they have no upper front incisor teeth, but have a tough, fibrous dental pad on the roof of their mouths. Lower teeth, tongue, and lips are used to chew food. From the mouth, food travels through the esophagus to the stomach.
The ruminant stomach is a multi-chambered organ that helps digest large amounts of fiber and cellulose material. The four chambers of the stomach are the rumen, reticulum, omasum, and abomasum. These chambers contain many microbes that break down cellulose and ferment ingested food. The abomasum is called the "true stomach" and is the equivalent of the monogastric stomach chamber where gastric juices are secreted. The four compartment gastric chamber provides ample space and microbial support necessary to digest plant material in ruminants. The fermentation process produces large amounts of gas in the stomach chamber, which must be eliminated. As in other animals, the small intestine plays an important role in nutrient absorption, and the large intestine helps in the elimination of waste.
Pseudo-ruminants (camels and alpacas) have a three-chamber stomach digestive system and a large cecum, a pouched organ at the beginning of the large intestine with microorganisms and is where the cellulose plant material is fermented and digested. The rumen is absent, but the omasum, abomasum, and reticulum are present in pseudo-ruminants.
The vertebrate digestive system has an oral cavity or mouth where food is ingested and chewed down or masticated with the teeth. The chemical process of digestion begins in the mouth. During food chewing, saliva is produced by salivary glands and mixes with the food. Saliva is a watery substance produced in the mouths of many animals. Three major glands secrete saliva: the parotid, submandibular, and the sublingual. Saliva contains mucus that moistens food and buffers the pH of the food. Saliva also contains immunoglobins and lysozymes, which have antibacterial action to reduce tooth decay by inhibiting growth of some bacteria. Saliva also has an enzyme called salivary amylase that begins the process of converting starches in the food into a disaccharide called maltose. Another enzyme called lipase is produced by cells in the tongue. Lipases are a class of enzymes that can break down triglycerides. The lingual lipase begins the breakdown of fat components in the food. The chewing and wetting action provided by the teeth and saliva prepare the food into a mass called the bolus for swallowing. The tongue helps in swallowing by moving the bolus from the mouth into the pharynx. The pharynx opens to two passageways: the trachea, which leads to the lungs, and the esophagus, which leads to the stomach. The trachea has an opening called the glottis, which is covered by a cartilaginous flap called the epiglottis. When swallowing, the epiglottis closes the glottis and food passes into the esophagus and not the trachea. This arrangement allows food to be kept out of the trachea.
The esophagus is a tubular organ that connects the mouth to the stomach and transports chewed foot to the stomach. The smooth muscles of the esophagus undergo a series of wavelike movements called peristalsis that push the food toward the stomach in one direction (irreversible). Peristalsis movement of the esophagus is an involuntary reflex that occurs as a result of swallowing food.
A ring-like muscle called the sphincter forms valves in the digestive system at the stomach end of the esophagus. In response to swallowing and the pressure exerted by the bolus of food, this sphincter opens, and the bolus enters the stomach. When there is no swallowing action, this sphincter is shut and prevents stomach contents from traveling up the esophagus. However, acidic digestive juices can escape into the esophagus sometimes and cause acid reflux or heartburn.
The stomach is where a large part of digestion occurs. The stomach is a saclike organ that secretes gastric digestive juices. Stomach pH ranges from 1.5 to 2.5 and this highly acidic environment is required for the chemical breakdown of food and the extraction of nutrients. The stomach can expand up to 20 times in size to allow food to enter and this expansion can help animals that eat large amounts of food when it is available.
In many animals, the stomach is the site of protein digestion. Protein digestion is mediated by an enzyme called pepsin in the stomach chamber. Pepsin is secreted by the chief cells in the stomach in an inactive form called pepsinogen. Pepsin breaks peptide bonds and cleaves proteins into smaller polypeptides. Pepsin also helps activate more pepsinogen, starting a positive feedback mechanism that generates more pepsin. Another cell type called parietal cells secrete hydrogen and chloride ions, which combine in the lumen to form hydrochloric acid, the primary acidic component of the stomach juices. Hydrochloric acid helps to convert the inactive pepsinogen to pepsin. The highly acidic environment also kills many microorganisms in the food and along with the pepsin enzyme, the result is the hydrolysis of protein in the food. Chemical digestion is facilitated by the churning action of the stomach. Contraction and relaxation of smooth muscles mixes the stomach contents about every 20 minutes. The partially digested food and gastric juice mixture is called chyme, which passes from the stomach to the small intestine. Further protein digestion takes place in the small intestine. Gastric emptying occurs within two to six hours after a meal. Only a small amount of chyme is released into the small intestine at a time. The movement of the chyme to the stomach into the small intestine is regulated by the pyloric sphincter.
When digesting protein and some fats, the stomach must be protected from getting digested by pepsin. Since pepsin is originally made in the inactive form pepsinogen, the stomach cells are protected. Also, the stomach has a thick mucus lining that protects the underlying tissue from the action of digestive juices. When this mucus lining is ruptured, wounds called ulcers can form in the stomach caused by bacteria.
Chyme moves from the stomach to the small intestine, which is the organ where digestion of proteins, fats, and carbohydrates is competed. The small intestine is a long tube-like organ with a highly folded surface containing finger-like projections called the villi. The apical surface of each villus has many microscopic projections called microvilli. These structures are lined with epithelial cells on the luminal side and allow for the nutrients to be absorbed from the digested food and absorbed into the bloodstream on the other side. The villi and microvilli, with their many folds, increase the surface area of the intestine and increase the absorption efficiency of the nutrients. Absorbed nutrients in the blood are carried into the hepatic portal vein, which leads to the liver. There, the liver regulates the distribution of nutrients to the rest of the body and removes toxic chemicals and pathogens.
The human small intestine is over 6 m long and is divided into three parts: the duodenum, the jejunum, and the ileum. The duodenum is the C-shaped, fixed part of the small intestine. The duodenum is separated from the stomach by the pyloric sphincter which opens to allow chyme to move from the stomach to the duodenum. In the duodenum, chyme is mixed with pancreatic juices in an alkaline solution rich in bicarbonate that neutralizes the acidity of chyme and acts as a buffer. Pancreatic juices also contain several digestive enzymes. Digestive juices from the pancreas, liver, and gallbladder, as well as from gland cells of the intestinal wall itself, enter the duodenum. Bile is produced in the liver and stored and concentrated in the gallbladder. Bile contains bile salts which emulsify lipids while the pancreas produces enzymes that catabolize starches, disaccharides, proteins, and fats. These digestive juices break down the food particles in the chyme into glucose, triglycerides, and amino acids. The bulk of chemical digestion of food takes place in the duodenum. Absorption of fatty acids also takes place in the duodenum.
The second part of the small intestine is called the jejunum, where hydrolysis of nutrients is continued while most of the carbohydrates and amino acids are absorbed through the intestinal lining. Some chemical digestion and the bulk of nutrient absorption occurs in the jejunum.
The ileum is the last part of the small intestine where the bile salts and vitamins are absorbed into the bloodstream. Undigested food is sent to the colon from the ileum via peristaltic movements (wave-like muscle contractions). The ileum ends and the large intestine begins at the ileocecal valve. The vermiform (worm-like) appendix is located at the ileocecal valve. The appendix of humans secretes no enzymes and has an insignificant role in immunity.
The large intestine reabsorbs the water from the undigested food material and processes the waste material. The human large intestine is much smaller in length compared to the small intestine but is larger in diameter. The large intestine has three parts: the cecum, the colon, and the rectum. The cecum joins the ileum to the colon and is the receiving pouch for the waste matter. The colon is home to many bacteria or "intestinal flora" that aid in the digestive processes. The colon can be divided into four regions: the ascending colon, the transverse colon, the descending colon, and the sigmoid colon. The main functions of the colon are to extract the water and mineral salts from undigested food, and to store waste material. Carnivorous mammals have a shorter large intestine compared to herbivorous mammals due to their diet.
The rectum is the terminal end of the large intestine and its primary role is to store the feces until defecation. The feces are propelled using peristaltic movements during elimination. The anus is an opening at the far end of the digestive tract and is the exit point for the waste material. Two sphincters between the rectum and anus control elimination: the inner sphincter is involuntary and the outer sphincter is voluntary.
Accessory organs of the digestive system add secretions (enzymes) that catabolize food into nutrients. Accessory organs include salivary glands, the liver, pancreas, and the gallbladder. The liver, pancreas, and gallbladder are regulated by hormones in response to the food consumed.
The liver is the largest internal organ in humans and it plays a very important role in digestion of fats and detoxifying blood. The liver produces bile, a digestive juice that is required for the breakdown of fatty components of the food in the duodenum. The liver also processes the vitamins and fats and synthesizes many plasma proteins.
The pancreas is another important gland that secretes digestive juices. The chyme produced from the stomach is highly acidic in nature. The pancreatic juices contain high levels of bicarbonate, an alkali that neutralizes the acidic chyme. Additionally, the pancreatic juices contain a large variety of enzymes that are required for the digestion of protein and carbohydrates.
The gall bladder is a small organ that aids the liver by storing bile and concentrating bile salts. When chyme containing fatty acids enters the duodenum, the bile is secreted from the gallbladder into the duodenum.
Nutrition and energy production in animals is needed for survival and animal nutrition is the source of materials needed for building DNA and other complex molecules needed for growth, maintenance, and reproduction. These processes are known as biosynthesis. Food source gives materials needed for ATP production in the cells. Animal diet must be balanced in order to provide vitamins and minerals needed for cellular functions.
Diet in humans and animals must be well balanced to provide nutrients needed for life, good health, and reproductive capability. Humans need fruits, vegetables, grains, proteins, and dairy.
Food sources provide necessary organic molecules for cells and tissues in the body. Carbohydrates or sugars are the primary source of organic carbons in the animal body. During digestion, digestible carbohydrates are broken down into glucose and used to provide energy through metabolic pathways. Complex carbohydrates, including polysaccharides, can be broken down into glucose through biochemical modification. However, humans do not produce the enzyme cellulase and lack the ability to derive glucose from polysaccharide cellulose. In humans, these molecules provide the fiber required for moving waste through the large intestine and a healthy colon. The intestinal flora in the human gut are able to extract some nutrition from these plant fibers. The excess sugars in the body are converted to glycogen and stored in the liver and muscles for later use. Glycogen stores are used to fuel prolonged exertions, such as long distance running, and to provide energy during food shortage. Excess glycogen can be converted to fats, which are stored in the lower layer of skin of mammals for insulation and energy storage. Excess digestible carbohydrates are stored by mammals in order to survive famine and aid in mobility.
Nitrogen is another requirement for nutrition. Protein catabolism provides a source of organic nitrogen. Amino acids are the building blocks of proteins and protein breakdown provides amino acids that are used for cellular function. The carbon and nitrogen derived from these become the building block for nucleotides, nucleic acids, proteins, cells, and tissues. Excess nitrogen must exerted because it is toxic. Fats add flavor to food and promote a sense of satiety or fullness. Fatty foods are also significant sources of energy because one gram of fat contains nine calories. Fats are required in the diet to aid in the absorption of fat-soluble vitamins and the production of fat soluble hormones.
Essential nutrients cannot be produced by the body or produced in enough quantity needed, and therefore these nutrients must be obtained from food outside the body. The omega-3 alpha-linolenic acid and the omega-6 linoleic acid are essential fatty acids needed to make some membrane phospholipids. Vitamins are another class of essential organic molecules that are required in small quantities for many enzymes to function and, for this reason, are considered to be coenzymes. Fat-soluble vitamins and water-soluble vitamins must be obtained from food. Minerals are inorganic essential nutrients that must be obtained from food. Minerals help in structure and regulation and are considered cofactors. Certain amino acids are essential must be obtained from outside the body. Of 20 required amino acids, the human body can only make 11 of these.
Water-soluble Essential Vitamins
Vitamin B1 (Thiamine)
Vitamin B2 (Riboflavin)
Vitamin B3 (Niacin)
Vitamin B5 (Pantothenic acid)
Vitamin B6 (Pyridoxine)
Vitamin B7 (Biotin)
Vitamin B9 (Folic acid)
Vitamin B12 (Cobalamin)
Vitamin C (Ascorbic acid)
Fat-Soluble Essential Vitamins
Vitamin A (Retinol)
Vitamin D
Vitamin E (Tocopherol)
Vitamin K (Phylloquinone)
Minerals Needed for the Body
Calcium
Chlorine
Copper
Iodine
Iron
Magnesium
Manganese
Molybdenum
Phosphorus
Potassium
Selenium
Sodium
Zinc
Essential amino acids that must be consumed
isoleucine
leucine
lysine
methionine
phenylalanine
tryptophan
valine
histidine
threonine
arginine
Essential amino acids anabolized by the body
alanine
selenocysteine
aspartate
cysteine
glutamate
glycine
proline
serine
tyrosine
asparagine
Animals need food to obtain food and maintain homeostasis, which is the ability of a system to maintain a stable internal environment even in the face of external changes to the environment, particularly body temperature.
The primary source of energy for animals is carbohydrates, mainly glucose. The carbohydrates are converted to glucose in the body through a series of biochemical reactions.
Adenosine Triphosphate, ATP, is the primary energy currency in cells. ATP stores energy in phosphate ester bonds. ATP releases energy when phosphodiester bonds are broken and ATP is converted to ADP and a phosphate group. ATP is produced by the oxidative reactions in the cytoplasm and mitochondrion of the cell, where carbohydrates, proteins, and fats undergo a series of metabolic reactions collectively called cellular respiration. For example, glycolysis is a series of reactions in which glucose is converted to pyruvic acid and some of its potential energy is converted to NADH and ATP.
ATP is required for all cellular functions. It is used to build the organic molecules that are required for cells and tissues. It provides energy for muscle contraction and for the transmission of electrical signals in the nervous system. When the amount of ATP is available in excess of the body's requirements, the liver uses the excess ATP and excess glucose to produce molecules called glycogen. Glycogen is a polymeric form of glucose and is stored in the liver and skeletal muscle cells. When blood sugar drops, the liver releases glucose from the stores of glycogen. Skeletal muscle converts glycogen to glucose during intense exercise. The process of converting glucose and excess ATP to glycogen and the storage of excess energy helps animals deal with mobility, food shortages, and famine.
Obesity is not healthy for humans and most animals, but some animals benefit from obesity, such as polar bears when food is scarce.
Digestive System Processes
Ingestion is the first step in the digestive process of breaking down food into smaller pieces in order to access the nutrients. Ingestion is taking in food from the mouth. In vertebrates, the teeth, saliva, and tongue play important roles in mastication (preparing food into bolus). As food is being mechanically broken down, the enzymes in saliva begin to chemically process the food as well. These processes break down food from large particles to soft mass that can be swallowed and travel the length of the esophagus.
Digestion and Absorption
Digestion is the mechanical and chemical breakdown of food into small organic fragments. Breakdown of macromolecules into smaller fragments of suitable size for absorption across the digestive epithelium is important. Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles such as simple sugar before they can be absorbed by the digestive epithelial cells. Different organs play specific roles in the digestive process. The animal diet needs carbohydrates, proteins, fats, as well as vitamins and inorganic components for nutritional balance.
Carbohydrates
The digestion of carbohydrates begins in the mouth. The salivary enzyme amylase begins the breakdown of food starches into maltose, a disaccharide. As the bolus of food travels through the esophagus to the stomach, no significant digestion of carbohydrates takes place. The esophagus produces no digestive enzymes but does produce mucus for lubrication. The acidic environment in the stomach stops the action of the amylase enzyme.
The next step of carbohydrate digestion takes place in the duodenum. Chyme from the stomach enters the duodenum and mixes with the digestive secretion from the pancreas, liver and gallbladder. Pancreatic juices also contain amylase, which continues the breakdown of starch and glycogen into maltose, a disaccharide.
The disaccharides are broken down into monosaccharides by enzymes called maltases, sucrases, and lactases, which are also present in the brush border of the small intestinal wall. Maltase breaks down maltose into glucose. Other disaccharides, such as sucrose and lactose, are broken down by sucrase and lactase, respectively. Sucrase breaks down sucrose (table sugar) into glucose and fructose, and lactase breaks down lactose (milk sugar) into glucose and galactose.
The monosaccharides (glucose) thus produced are absorbed and then can be used in metabolic pathways to harness energy. The monosaccharides are transported across the intestinal epithelium into the bloodstream to be transported to the different cells in the body.
Protein
A large part of protein digestion occurs in the stomach. The enzyme pepsin has an important role in the digestion of proteins by breaking down the intact protein to peptides, which are short chains of four to nine amino acids. In the duodenum, other enzymes like trypsin, elastase, and chymotrypsin act on the peptides reducing them to smaller peptides. Trypsin, elastase, carboxypeptidase, and chymotrypsin are produced by the pancreas and released into the duodenum where they act on the chyme. Further breakdown of peptides to single amino acids is aided by enzymes called peptidases (those that break down peptides). Specifically, carboxypeptidase, dipeptidase, and aminopeptidase play important roles in reducing the peptides to free amino acids. The amino acids are absorbed into the bloodstream through the small intestines.
Protein digestion summary:
The liver regulates distribution of amino acids to the rest of the body. In the stomach, pepsin breaks down proteins into fragments, called peptides. Protein-digesting enzymes are secreted from the pancreas into the small intestine. Amino acids are absorbed from the small intestine into the bloodstream. In the small intestine, a variety of enzymes break large peptides into smaller peptides, and then into individual amino acids. A small amount of dietary protein is lost in the feces.
Lipids
Lipid digestion begins in the stomach with the aid of lingual lipase and gastric lipase. However, the majority of lipid digestion occurs in the small intestine due to pancreatic lipase. When chyme enters the duodenum, the hormonal responses trigger the release of bile, which is produced in the liver and stored in the gallbladder. Bile aids in the digestion of lipids, primarily triglycerides by emulsification. Emulsification is a process in which large lipid globules are broken down into several small lipid globules. These small globules are more widely distributed in the chyme rather than forming large aggregates. Lipids are hydrophobic substances: in the presence of water, they will aggregate to form globules to minimize exposure to water. Bile contains bile salts, which are amphipathic, meaning they contain hydrophobic and hydrophilic parts. Thus, the bile salts hydrophilic side can interface with water on one side and the hydrophobic side interfaces with lipids on the other. By doing so, bile salts emulsify large lipid globules into small lipid globules.
Emulsification is important for digestion of lipids. Pancreatic juices contain enzymes called lipases (enzymes that breakdown lipids). If the lipid in the chyme aggregates into large globules, very little surface area of the lipids is available for the lipases to act on, leaving lipid digestion incomplete. By forming an emulsion, bile salts increase the available surface area of the lipids by many fold. The pancreatic lipases can then act on the lipids more efficiently and digest them. Lipases break down the lipids into fatty acids and glycerides. These molecules can pass through the plasma membrane of the cell and enter the epithelial cells of the intestinal lining. The bile salts surround long-chain fatty acids and monoglycerides forming tiny spheres called micelles. The micelles move into the brush border of the small intestine absorptive cells where the long-chain fatty acids and monoglycerides diffuse out of the micelles into the absorptive cells leaving the micelles behind in the chyme. The long-chain fatty acids and monoglycerides recombine in the absorptive cells to form triglycerides, which aggregate into globules to become coated with proteins. These large spheres are called chylomicrons. Chylomicrons contain triglycerides, cholesterol, and other lipids and have proteins on their surface. The surface is also composed of the hydrophilic phosphate "heads" of phospholipids. Together, they enable the chylomicron to move in an aqueous environment without exposing the lipids to water. Chylomicrons leave the absorptive cells via exocytosis. Chylomicrons enter the lymphatic vessels, and then enter the blood in the subclavian vein.
Vitamins can be either water-soluble or lipid soluble. Fat soluble vitamins are absorbed in the same manner as lipids. It is important to consume some amount of dietary lipid to aid the absorption of lipid-soluble vitamins. Water-soluble vitamins can be directly absorbed into the bloodstream from the intestine.
Elimination is the final step in digestion which involves the elimination of undigested food content and waste products. The undigested food material enters the colon, where most of the water is reabsorbed. The colon is also the home of microflora called "intestinal flora" that aid in the digestion process. The semi-solid waste is moved through the colon by peristaltic movements of the muscle and is stored in the rectum. As the rectum expands in response to storage of fecal matter, it triggers the neural signals required to set up the urge to eliminate. The solid waste is eliminated through the anus using peristaltic movements of the rectum.
Diarrhea and constipation are some of the most common health problems with elimination. Constipation is a condition where the feces are hardened because of excess water removal in the colon. In contrast, if enough water is not removed from the feces, the result is diarrhea. Many bacteria, including the ones that cause cholera disease, affect the proteins involved in water reabsorption in the colon and result in excessive diarrhea.
Emesis (vomiting) is the elimination of food from the body by forceful expulsion from the mouth. Emesis is often in response to an irritant that affects the digestive tract, including but not limited to viruses, bacteria, emotions, sights, and food poisoning. This forceful expulsion of the food is due to the strong contractions produced by the stomach muscles. The process of emesis is regulated by the medulla (lower brain stem).
Digestive System Regulation
The brain is the control center in animals and humans for the sensation of hunger and satiety. The functions of the digestive system are regulated through neural and hormonal responses.
The first neural response to food is the reaction of smell, sight, or thought of food in the form of salivation. The salivary glands secrete more saliva in response to stimulation by the autonomic nervous system triggered by food in preparation for digestion. Simultaneously, the stomach begins to produce hydrochloric acid to digest the food. Peristaltic movements of the esophagus and other organs of the digestive tract that help food move along are under control of the brain. The brain prepares these muscles for movement as well. When the stomach is full, the part of the brain that detects satiety signals fullness. There are three overlapping phases of gastric control: the cephalic phase, the gastric phase, and the intestinal phase. Each of these requires many enzymes and is under neural control as well.
Digestive phases of gastric control begin with the cephalic phase, which is controlled by the neural response to the stimulus provided by food. Sight, sense, and smell trigger the neural responses resulting in salivation and secretion of gastric juices. The gastric and salivary secretion in the cephalic phase can also take place due to the thought of food. The central nervous system prepares the stomach for food.
The gastric phase begins once food arrives in the stomach and builds on the stimulation provided during the cephalic phase. Gastric acids and enzymes process the ingested materials. The gastric phase is stimulated by distension of the stomach, a decrease in pH of the gastric contents, and the presence of undigested material. This phase consists of local, hormonal, and neural responses. These responses stimulate secretions and powerful contractions.
The intestinal phase begins when chyme (gastric juices and partially digested food) enters the small intestine triggering digestive secretions. This phase controls the rate of gastric emptying. In addition to gastric emptying, when chyme enters the small intestine, it triggers other hormonal and neural events that coordinate the activities of the intestinal tract, pancreas, liver, and gallbladder.
The hormonal responses to food occur as the endocrine system controls the response of the various glands in the body and the release of hormones at the appropriate times.
One of the most important factors under hormonal control is the stomach acid environment. During the gastric phase, the hormone gastrin is secreted by G cells in the stomach in response to the presence of proteins. Gastrin stimulates the release of stomach acid, or hydrochloric acid (HCl) which aids in the digestion of the proteins. However, when the stomach is emptied, the acidic environment need not be maintained and a hormone called somatostatin stops the release of hydrochloric acid. This is controlled by a negative feedback mechanism.
In the duodenum, digestive secretions from the liver, pancreas, and gallbladder play an important role in digesting chyme during the intestinal phase. In order to neutralize the acidic chyme, a hormone called secretin stimulates the pancreas to produce alkaline bicarbonate solution and deliver it to the duodenum. Secretin acts in tandem with another hormone called cholecystokinin (CCK). Not only does CCK stimulate the pancreas to produce the requisite pancreatic juices, it also stimulates the gallbladder to release bile into the duodenum.
Another level of hormonal control occurs in response to the composition of food. Foods high in lipids take a long time to digest. A hormone called gastric inhibitory peptide is secreted by the small intestine to slow down the peristaltic movements of the intestine to allow fatty foods more time to be digested and absorbed.
Hormone control of the digestive system and the role of each hormone is an important area of ongoing research.